1. Field of the Invention
The present invention relates in general to a polypropylene recloseable zipper, and in particular to a polypropylene recloseable zipper for flexible bags or packages that are resealable.
2. Description of the Related Art
The term xe2x80x9czipperxe2x80x9d as used herein is intended to encompass any closure arrangement for a bag or package. These bags are used in a wide variety of consumer packaging applications and are usually made from a polymeric or thermoplastic material. The zipper is usually located on one side near the edge and has two strips with profiles, such as complementary projections and grooves, that are constructed to interlock with proper alignment. The recloseable zipper allows the bag to be resealable when contents are removed or added. The zipper is preferably flexible as is the bag and is firmly attached to the bag. While fusible attachment is the preferred attachment, there exist other ways of attaching the zipper to the bag, for example, by way of an adhesive or some anchoring means.
A thermal fusion process, also referred to herein as heat welding or simply welding, permanently attaches both strips making up the recloseable zipper to the bag or polymeric film used in forming the bag. The zipper attachment is often made prior to sealing and cutting of the film""s side walls to form the finished bag.
As is known in this industry, a proper fused attachment of the zipper to the bag ensures seal tightness and avoids any separation of the zipper from the bag.
Polyethylene is widely used in resealable bags or packages and is usually employed both for the bag or package film material as well as for the extruded strips which make up the zipper. The polyethylene material extrudes easily, is flexible, and heat welds to the film material at a reasonable temperature. Thermal fusion of zippers to film material is done in a manner to provide sufficient heat to bond the zipper to the film, yet not at a temperature that causes thermal deformation of the zipper or film material.
It is further known in the industry that some materials are fusibly incompatible. While polyethylene may be fused to polyethylene, it cannot be directly fused to polypropylene. Some packaging applications require a polypropylene film material to function as a more active fluid barrier. In order to fuse or heat seal polyethylene to polypropylene, the industry practice is to place a tie layer of ethylene vinyl acetate (EVA), or some other accelerant, to the weld area of the polypropylene film. The EVA acts as the tie layer for welding the polyethylene zipper to the polypropylene film. The co-extrusion process associated with making those materials of the prior art approach is expensive, time consuming, and requires application of the accelerant to a large weld area for the process to work effectively.
U.S. Pat. No. 4,807,300 recognizes the problems of fused attachment of the zipper strips to a fusibly incompatible material and addresses this problem with a different approach. This patent describes a particular anchor-socket arrangement. Fused connecting anchors extend through holes in the bag material.
There still exists a need for a polypropylene closure arrangement or recloseable zipper which is sufficiently flexible to be extruded in a similar manner as polyethylene, and still allow the recloseable zipper to be directly fused or heat welded to the polypropylene film for making a resealable polypropylene package.
Accordingly, an object of the present invention is to provide a polypropylene zipper that may be attached by heat welding or fusion directly to the polypropylene film.
Another object of the present invention is to provide a polypropylene zipper with sufficient flexibility during the processing to be extruded and still have the ability to heat weld to a polypropylene film at reasonable temperatures without a tie layer or accelerant.
Another object of the present invention is to provide an extruded polypropylene zipper fabricated from a polypropylene random copolymer base resin.
Another object of the present invention is to provide a recloseable zipper which uses a homogenized blend of resin component which is directly weldable to polypropylene film.
Still another object of the present invention is to provide a closure arrangement for a resealable polypropylene bag that includes strips smaller in width than a polyethylene closure arrangement having a tie layer or accelerant co-extruded therein.
The above and other objects are accomplished with a polypropylene zipper comprising a blend of polypropylene, a flexible modifier, and a slip component. Preferably, the polypropylene zipper comprises a blend of about 78% on a weight percent basis polypropylene, about 20% on a weight percent basis flexible modifier, and about 2% on a weight percent basis slip component. The polypropylene is preferably a random copolymer polypropylene. The flexible modifier is preferably an ethylene copolymer. The slip component preferably includes an oleamide functioning as a slip additive in a carrier resin of low density polyethylene (LDPE).
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is described and illustrated.